1. Field of the Invention
The present invention relates to the field of first-in first-out (FIFO) buffer memories.
2. Prior Art
First-in first-out buffer memories are dual port memories having characteristics which are highly useful in various applications. In particular, such memories allow the writing of data to the memory and the reading of data from the memory simultaneously, and at independent rates limited only by the speed capability of the device in question. To prevent the overflow and loss of data when writing to the memory, and alternatively the reading of false data when reading the memory, full and empty flags are commonly provided by such memories to halt the writing and reading operations, respectively, until the condition giving rise to the flag has passed by the further reading from or writing to the memory, respectively. An example of this is the Full Flag FF and the Empty Flag EF of the MK4501, a 512 X 9 FIFO previously manufactured by Mostek. See also the article on this device entitled "FIFO--The Glue Holding Systems Together" starting on page 181 of the June 1983 issue of Computer Design magazine.
In certain applications however, the full and empty flags may not provide a system designed with the full flexibility needed, as it may be inconvenient, illogical, or even impossible to immediately stop the writing or reading sequences of the equipment connected to the FIFO buffer memory. Thus, for this purpose, it is desirable to have some additional flags to provide an indication of an almost full and almost empty status to provide a warning, whereby writing and reading operations may be more conveniently terminated or initiated as the case may be, before data loss or false data is obtained. Further, a half full flag is also useful to generally indicate the status of the memory to provide an even more advanced warning for write or read operations.